Game changing battery health monitoring for EVs enters UK production

The sensor has been developed using ISO26262 processes and tested to ISO Automotive Standards by an independent test house, Applus+ 3C Test and in-house test capability.

Manufacturing of the Production version of the Battery Safety Sensor begins in the UK under strict quality-controlled conditions to satisfy orders from a rapidly growing number of OEMs and Tier 1s where they are being used in ASIL B applications.

Metis Engineering Managing Director Joe Holdsworth said: “Our Battery Safety sensor measures pressure change and Volatile Organic Compounds (VOC’s) for the detection of cell venting, which is an early stage of thermal runaway. It also monitors Humidity, Dew Point and Air Temperature parameters required to ensure that the battery continues to operate in optimum conditions as well as many other useful features such as a low power mode and configurable CAN settings”.

He continued. “Thankfully the occurrences of EV battery fires are very rare, but when they do occur, they are usually catastrophic for the vehicle. As cells age the chance of one in a pack of hundreds or thousands going bad increases significantly. The early detection of cell venting is vital to the safety of the vehicle’s passengers and everything in the immediate proximity, we hope that this sensor offers an affordable part of the solution.”

In April 2022 Metis released the first Beta versions of the new streamlined design, optimised for manufacturing in the high volumes required by automotive OEMs. Beta units were tested by a number of EV OEMs and battery manufacturers and their feedback informed the minor changes implemented in the units now certified to ISO Automotive Standards.

The unique sensor is designed to pick up a range of environmental parameters required to ensure that the battery continues to operate in optimum conditions, including VOCs, Pressure change, Humidity, Dew point and has an optional Accelerometer to record shock loads. This data can be used to crosscheck with other inputs, such as cell temperatures, to check for cell venting. The sensor relays the data over a configurable CAN interface to a control unit, such as the vehicle’s ECU, to alert the driver that cell venting has occurred. The sensor can also trigger the process to cut the circuit to the battery pack, giving it opportunity to cool down with the objective of preventing thermal runaway.  

Limitations of current battery management systems

EV battery packs will already have a Battery Management System (BMS) fitted. As well as managing the charging and discharging of the packs, they also attempt to monitor the health of the battery pack, although this is typically limited to temperature sensors, one for every few cells and by monitoring for voltage changes. This system works fine if it happens to be the cell with the temperature sensor that goes bad, but if the cell is a distance away from the sensor in the pack, by the time the sensor registers the change in temperature, if at all, it would very likely be too late. Detecting cell issues through voltage fluctuations can also be difficult in the short term because other cells in parallel can prop up the voltage, disguising issues with a cell.

The Metis Battery Safety Sensor typically detects venting within seconds.

There are several stages to a cell going into thermal runaway. First, there is an increase in temperature, often caused when a battery is overworked, has a manufacturing fault, or is aged. The rise in temperature causes the pressure inside the battery cell to rise. Eventually that pressure, if high enough, causes venting from the sealed battery cell. If load is not removed from the cell, then a thermal runaway can occur.

If a battery is driven to failure after venting, it can catch fire in a self-oxidising reaction, which will most likely result in a runaway reaction, setting off the cells around it, and causing the total loss of the vehicle.

The Metis Battery Safety Sensor also includes an optional Accelerometer, so it can monitor shock loads up to 24G and impact duration that the battery pack may experience. Should the EV be involved in a collision, the sensor will show whether the pack experienced loadings above safe levels and for what duration. This can inform the decisions as to whether the battery pack requires a service, a second life in a domestic or industrial Energy Storage System (ESS) or be sent for recycling and any subsequent insurance claims.  

Battery packs get warm when they are charged or discharged. To prevent overheating, most packs are cooled in some way. If they are cooled below the ambient temperature, they can dip below the dew point (the temperature at which the air can’t carry moisture and it condenses onto cooler surfaces), which could lead to shorting and thermal incidents. The Metis Battery Safety Sensor will monitor relative humidity as well as the dew point in the battery pack and can trigger a warning before condensation settles on the battery or other components.